In domestic communication satellite systems, which interconnect large numbers of very small aperture earth terminals, the most important parameters affecting the system capacity are the Effective Isotropic Radiated Power (EIRP) and the available bandwidth. EIRP refers to a measure of the satellite's transmitter power which takes into consideration the gain of the antenna. EIRP is the power of a transmitter and isotropic antenna that would achieve the same result as the transmitter and antenna which is actually employed.
In the past, high antenna gain and multiple frequency reuse has been achieved by employing a plurality of up and down link beams covering the regions of a country or other areas of the earth to be served. Both frequency division and time division systems have been used or proposed to interconnect large numbers of signals from many geographically separated earth stations. Time division systems permit the satellite transmitters to operate efficiently. This is because only one time division signal at a time is amplified in a transmitter, so it may be operated at or close to signal channel saturation, the most efficient operating point. However, time division systems require high power ground transmitters and expensive signal processing and are therefore incompatible with low cost earth stations. Frequency division systems are better suited to low cost earth stations, but have lower satellite transmitter efficiency because each transmitter handles multiple carriers. Since multiple carrier amplifiers generate undesirable intermodulation products that increase in power as the transmitter efficiency is increased, the optimum compromise between transmitter efficiency and intermodulation product generation results in a relatively low transmitter efficiency.
In Ku band, the satellite communication band most suitable for two-way service between very small terminals, the attenuation of the signals by rain is an important consideration in the design of the system. In the previous systems, this attenuation is overcome on the downlink by using higher satellite transmitter power per channel than would be necessary for clear weather service, typically four times as much. This accommodation of rain attenuation therefore results in more expensive satellites having fewer available channels.
The available bandwidth of a satellite system is determined by the number of times the allocated frequency spectrum can be reused. Polarization and spatial isolation of beams have been employed to permit reuse of the frequency spectrum. As the number of isolated beams is increased, however, the problem of interconnecting all the users becomes very complicated and is one of the factors that limit the number of reuses of the frequency spectrum.
The present invention is directed toward overcoming each of the deficiencies mentioned above.